This invention relates to split multihole spinnerets for the production of multiple yarn ends and, more particularly, to a method and apparatus for diagnosing and correcting non-uniformities in construction of split multihole spinnerets to thereby control denier split in multiple end spinning while employing only one metered feed stream.
Throughout the present specification and claims, the term "yarn" is employed in a general sense to indicate a continuous, often plied, strand comprised of fibers, filaments, glass, metal or plastic. An "end" is a single such strand of yarn. A "split multihole spinneret" is a spin pot spinneret which is divided into two or more parts by means of an unpierced stripe or stripes wide enough to form a visible split between the multiple yarn ends below the spinneret. By "unpierced stripe" is meant that no spinneret holes are drilled in a narrow area across the face of the spinneret. As used herein, the term "fluid" indicates any substance exhibiting laminar flow characteristic. In this discussion, a "Newtonian fluid" is one that under given conditions has a viscosity which is constant and independent of the rate of shear; conversely, a "non-Newtonian fluid" is one that exhibits an apparent viscosity which varies with the rate of shear.
The conversion from single end spinning to multiple end spinning can be extremely expensive if special equipment is required to meter and segregate each yarn end. Aside from initial equipment costs there is the added factor of restricted equipment space. A simple method of reducing these limitations is to feed one metered stream and determine the flow split by the hydraulics of the fluid and the geometry of the split multihole spinneret. This method of multiple end spinning has a further advantage in that a single spin pot and metering system design can be used for spinning both single and multiple ends, thereby increasing the operational flexibility of the equipment. Uniform fluid properties and dynamic similarity between ends in the split multihole spinneret are prerequisites for adequate denier control. Ideally, end to end variations in denier or flow rate are small enough to be inconsequential; in practice, however, there are often relatively wide denier differences between ends when using spinnerets produced to normal tolerances by conventional manufacturing processes.
Some of the factors contributing to denier variability in both single and multiple end spinning are: incorrect spin pump settings, temperature profile across the spinneret face, filament to filament variability due to the range of tolerable hole dimensions within the spinneret, random unfiltered sludge particles variable across and within a given yarn end, air differential in the quench chamber, combinations of these factors, etc. An additional factor, peculiar to multiple end spinning, is filament crossover from end to end as the filaments are extruded from the split multihole spinneret. Denier variability can be reduced by the periodic cleansing of process equipment and by the installation of adequate monitoring equipment; research is continuing to further decrease denier differences.
One of the more important facets dealt with in the prior art is the metering accuracy of spinnerets. Various methods have been proposed for determining the metering accuracy of holes, such as that disclosed by Levy in U.S. Pat. No. 1,676,831 wherein fluid flow rate is measured by first passing a fluid at a given pressure through a hole, and then collecting and weighing the fluid at fixed time intervals. It has also been recognized by Hitchner in U.S. Pat. No. 2,925,692 that the measurement of back pressure exerted due to the resistance to flow of air through a capillary is a means of obtaining metering accuracy. Air is not suitable for measuring the metering accuracy of some spinneret holes because in the range of pressures that are useful for measuring flow resistance accurately, the air flow is turbulent rather than having the laminar flow characteristics of the fluid to be spun. The resultant measurements vary widely and, as a consequence, are not accurate enough for a determination of acceptable denier variation, particularly in fiber forming polymers such as nylon and polyester. Additionally, Booy et al. teach in U.S. Pat. No. 3,433,055 another method of obtaining metering accuracy whereby the holes of a spinneret plate are sequentially positioned under a valve supplied with pressurized liquid and then injected with the liquid. The back pressure due to the flow resistance of each hole is measured and compared to the back pressure generated by a previously tested standard hole to determine deviation in flow resistance of the hole, and finally, the liquid flow is interrupted at the valve to maintain the system under pressure for purging the next capillary too be tested.
Adequate end to end denier control could be achieved by testing all of the individual holes by a prior art method. However, the teachings of the prior art are both costly and time-consuming as spinnerets may have hundreds of holes to be inspected, and in multiple end spinning where the same number of filaments per yarn end may be desirable, this figure can be multiplied by a constant factor. A solution to this problem would be to eliminate the necessity of inspecting all of the holes in a spinneret plate. We have now discovered a novel method of doing this in the extrusion of multiple ends from a split multihole spinneret plate fed by a single metered stream.